Many long-standing questions about fundamental mechanisms of neurotransmission recur in current attempts to understand how channelopathies give rise to synaptic defects. A striking overlap between cell biological and pathophysiological questions arises in the case of P/Q-type voltage-gated Ca2+ channels, whose pore-forming alpha1A subunit (also called alphaCa-v2.1) plays a dominant role in supporting voltage-gated presynaptic Ca2+ entry and fast excitatory and inhibitory neurotransmission. Working together with N- and R-type Ca2+ channels, P/Q-type channels contribute the majority of presynaptic Ca2+ entry and also emerge as the principal molecular target in known calcium channelopathies, including monogenetic forms of such neurological disorders as ataxia, epilepsy and headache. This project will investigate fundamental issues about P/Q-type channels and neurotransmission that may also illuminate pathophysiological mechanisms. Working with dissociated neuronal cultures from alpha1A -/- mice, we will examine the ability of various mutant human CCIA subunits to support Ca2+ channel activity and synaptic transmission. We will test the hypothesis that the relative efficacy of P/Q-type channels is governed by P/Q-selective "slots" that cap the contribution of P/Q-type channels even when they are greatly overexpressed. Specific questions to be addressed include the following: Do type selective slots for presynaptic N- and R-type Ca2+ channels co-exist along with those functionally defined for P/Q channels? If type-specific slots exist, does deficiency in Ca2+ channel function translate into an overall reduction in nerve terminal Ca2+ influx? What is the basic topography of Ca2+ channels in small presynaptic terminals? Where does the competition for slots occur and what is its molecular basis? What are the cell-biological or biophysical mechanisms by which human disease mutations affect Ca2+ channel function and higher-order pathophysiology? Why do P/Q channel diseases show a dominant negative inheritance? What are the homeostatic mechanisms that compensate for loss of P/Q channel function and what is their role in governing the overall outcome of the disease?